A number of assay devices for use in the measurement of one or more analytes in a liquid sample are known. Some of these are complex devices for use by trained personnel; for example, sampling devices for industrial liquids, water etc. or laboratory tests on blood, urine and the like. However, an important category of assay devices includes those intended for home use by consumers, for example home pregnancy test kits. Home pregnancy test kits typically function by determining the amount of the hormone human chorionic gonadotrophin (hCG) in a urine sample, and normally provide a visible indication as to whether a subject is pregnant.
The specific example of the Clearblue test from Unipath, England (Clearblue is a registered Trade Mark) as described in EP 291194 and EP 560411 will now be used to illustrate various issues concerning such devices, although the issues are applicable to a broad class of assay devices.
EP 291194 discloses a lateral flow assay device whereby sample is introduced onto a porous carrier such as nitrocellulose which has been pretreated with a mobilisable labelled first species such as an antibody capable of binding with the analyte of interest. The labelled analyte-antibody complex then permeates into an analyte detection region whereupon it reaches a second species (antibody), immobilised on the porous carrier, which binds the complex. Typically the second species is immobilised in a stripe across the carrier such that, in the presence of analyte, a test signal in the form of a line will appear in the analyte detection region due to concentrating of the label, typically gold or dyed latex.
Such assay devices are used amongst others by users wishing to tell whether they are pregnant, i.e. to detect the presence of the pregnancy hormone hCG. Such users may be using the assay device for the first time or may be a highly emotional state. Furthermore, the test will normally be carried out and interpreted by someone who typically will not be clinically trained. Thus, it is advantageous to have an assay device which is both accurate and reassuring to the user, providing as little ambiguity as possible.
In order to ascertain whether the assay has been carried out properly, a control region is typically present. For example, in EP 291194, an immobilised species is present which is able to bind to the labelled first species. Thus, the control region serves to indicate that in the absence of a positive result, the assay device was both functioning properly and used correctly. It thus indicates that enough sample had been added and that the labelled species was present and was able to permeate along the porous carrier, i.e. that there are no blockages or manufacturing defects etc. The control region is typically downstream of the test region. However, the strength of the signal formed at the control region will depend upon the level of analyte present. A high level of analyte will arguably bind more of the mobilised species leaving less unbound species to reach and bind at the control line.
EP 355244 (Abbott Laboratories) discloses such a control region (e.g. denoted by reference numeral 32 in FIG. 1c) which is configured as a minus sign (see claim 3 and FIGS. 1b and 1c) perpendicular to the direction of flow. The minus sign is also designed to bisect the read (vertical) line such that, in the absence of analyte, only a minus sign is formed, and in the presence of analyte, a plus sign is formed.
However, there are drawbacks with using this approach, one of which is that when using a capture zone such as a line disposed parallel to the direction of flow, binding of the labelled species takes place preferentially at the leading front edge. A strong signal will be observed at the “beginning” of the line which will fade out along its length. This is undesirable from the point of view of a consumer, as the consumer will want to be reassured by the presence of a bold and unambiguous line.
As an alternative, EP 421294 discloses an offset symbol (see FIG. 6). In such an embodiment the test signal and control signal are produced at an angle (denoted by reference letter “C” in FIG. 6) to the direction of fluid flow overcoming the problem of signal fade out. This however is something of a compromise since an offset cross may not be instantly recognisable as a “plus” sign indicative of a positive result, nor would an angled line be necessarily recognisable as a “minus” sign indicative of a negative result.
In the case of EP 291194, the control signal may be a line perpendicular to the direction of flow, which resembles the test signal. In the presence of analyte, two vertical lines are seen. However, in the absence of analyte, only the control signal line is seen, i.e. no visible signal is formed in the analyte detection region. The inventors have found that the absence of a line of any sort in the analyte detection region, following performance of the assay, is undesirable to consumers.